1. Field of the Invention
The present invention relates to a communication device having a traffic shaping function, and more specifically to an ATM terminal and an ATM switching system each having a traffic shaping function for a fixed length packet, and a communication device having a traffic shaping function for a variable length packet.
2. Description of the Related Art
An asynchronous transfer mode (ATM) technique for efficiently supporting various communication traffics such as voice, images, and data by using fixed length packets each called cell is widely known as a communication technology suitable for multimedia communications.
In the ATM, a user makes a contract about a transmission bandwidth with a network before cell transmission in units called VC (Virtual Connection) in advance. The contents of the contract differs according to the type of information to be transferred through each VC referred to above. In a real-time traffic for communications of an audio signal and a picture signal that demands for a low delay transfer, for example, a constant bandwidth is ensured within a line in advance and each cell is sent within the ensured bandwidth.
In “The ATM Forum TM4.0” (prior art 1), a CBR (Constant Bit Rate) service category in which cells are sent within a fixed bandwidth ensured in advance according to a contract, and a VBR (Variable Bit Rate) service category in which the maximum transmission bandwidth and the average transmission bandwidth are placed under contract and cells are sent in a transmission bandwidth which varies according to the elapse of time, are shown as service classes for transferring real-time traffic data for speech and pictorial communications.
On the other hand, a computer-to-computer data communication has a problem in that when a bandwidth necessary for data transmission is ensured within a line in advance, the preserved bandwidth is occupied needlessly during a data transmission-free period because a traffic occurs on a burst basis (unexpectedly) and is terminated. The bandwidth held by the transmission line cannot be utilized effectively. In general, a burst traffic for non-real time communications does not attach much importance to the transmission delay important for communications by voice or the like. Thus, in the burst traffic communication, the transmission of each data cell is started without ensuring a required bandwidth in advance and the cells are temporarily stored at a node on which large amounts of data cells are focused. The bandwidth for the transmission line is effectively utilized. At each node, a cell for the real-time traffic is outputted in preference to a cell for the burst traffic. When the amount or rate of cells stored at the node exceeds the buffer capacity of the node, some of data cells belonging to the subsequent-incoming burst traffic are discarded.
In the prior art 1, an ABR (Available Bit Rate) service category for feeding back a congested (busy) state in a network to a cell sending terminal to thereby prevent the discarding of cells and a UBR (Unspecified Bit Rate) service category for allowing a sending terminal to continuously transmit cells so long as an empty or vacant bandwidth exists in a line in order to effectively utilize a line bandwidth are given as service classes for transferring the burst traffic cells. VC multiplexed over an ATM transmission line falls into or belongs to any of the service classes of said four categories.
From the characteristics about the delay/discarding of the respective service classes, each cell, which belongs to VC placed under the CBR service, is transferred most preferentially when traffics of a plurality of VC compete with one another at a node in the network. Priority control on the cell transfer is performed in the order of the CBR, VBR, ABR and UBR services.
A terminal (user), which takes advantage of the respective CBR, VBR and ABR services, makes a contract with a network, which is related to a transmission bandwidth such as the maximum transmission bandwidth, prior to its communication on the network. In the communication of the UBR service class there are two cases: one in which a user makes a contract related to the transmission bandwidth (e.g., the maximum transmission bandwidth) and another in which no contract is done. In a public ATM network, a flow rate of cells transmitted from a terminal is monitored at an entrance of the network so as to detect VC which violates a contracted bandwidth. A marking indicative of each cell to be discarded upon the occurrence of congestion, for example, is effected on each cell which belongs to the violated VC. Such monitoring of each cell placed under violation of the contract is called UPC (Usage Parameter Control).
When a cell is discarded in the course of a transmission line, it is normally recognized by the receiving terminal that the cell has been discarded. Since, in this case, the receiving terminal requests each of transmitting terminals to re-send the discarded information and hence the transmitting terminal re-sends the information, the finally failure-free information can be sent to the corresponding receiving terminal. However, the discarding of cells in the ATM network could lead to a great delay in the transmission of the information and the occurrence of the secondary additional congestion by the re-sent cells. Thus, it is important that each transmitting terminal controls the transmission of cells at a cell flow rate set according to each contracted bandwidth so that the cells are not determined as the violated cells by the UPC function held by the network. Such a cell flow-rate control function is called traffic shaping. Incidentally, the traffic shaping is necessary even for respective switching nodes or switching systems located at important points in the ATM network to accommodate or absorb fluctuations of a cell interval for each VC, which occur in the network.
As a communication device or apparatus provided with the aforementioned traffic shaping function, there is known, for example, “Cell flow controller and ATM communication network” described in Japanese Unexamined Patent Publication No. Hei 6-315034 (prior art 2). When a relay node in an ATM network sends out each received cell entering with a bandwidth exceeding a contracted value to an output line with the same bandwidth as upon its reception, the cells are excessively sent out to the output line, thus causing a possibility that the cells will be discarded due to the UPC function at other nodes on a transmission route or line. Therefore, the prior art 2 has proposed that a memory for temporarily storing cells is provided in the cell flow controller and the cells received over the contracted bandwidth are temporarily stored in the memory and read out in accordance with the contracted bandwidth. However, while the prior art 2 has been described about a cell sending-interval control function using a buffer memory, which is necessary at the minimum to perform traffic shaping, it does not disclose such information as a specific device configuration. the required capacity of memory, and a process time, which is necessary for implementing the invention.
As another known technique related to the traffic shaping, there is known “Traffic shaping device” described in Japanese Unexamined Patent Publication No. Hei 9-307566 (prior art 3). In the prior art 3, the estimated sending time of the next cell is managed by a binary tree structure for each VC, and VC for which cells are transmitted in top priority is selected in a short processing time of the order of log 2 [number of VC] by special sorting using the result of previous sorting. According to the prior art 3, the time required to select a cell to be next transmitted can be greatly shortened using a memory having reduced capacity.
As bandwidth contracts for taking advantage of a public ATM network, there are known a contract made in VC units and a contract made in units of VP (Virtual Path) comprised of bundles of a plurality of VC. The bandwidth contract given in the VP units is normally made for each positions of the partners to communicate therewith. When a terminal 300 communicates with terminals 1901, 1902 and 1903 through a public ATM network 340 as shown in FIG. 19 by way of example, bandwidth contracts are made for each VP for connecting the terminal 300 to the partner terminals. Namely, for example, a contract related to a bandwidth of 90 Mbps, a contract related to a bandwidth of 30 Mbps and a contract related to a bandwidth of 30 Mbps are respectively made to VP(0)210 lying between the terminal 300 and the terminal 1901, VP(1)220 lying between the terminal 300 and the terminal 1902, and VP(2)230 lying between the terminal 300 and the terminal 1903. These VP(0) through VP(2) are tied up in a bundle on a line of 150 Mbps connecting between the terminal 300 and an ATM switch 341.
FIG. 2 is a conceptional diagram showing the relationship between a line 200 lying between the terminal 300 and the ATM switch 341 shown in FIG. 19 and VP and VC multiplexed over the line.
In FIG. 2, three VC 211, 212 and 213 are tied up in a bundle within VP(0) 210 of 90 Mbps, two VC 221 and 222 are tied up in a bundle within VP(1) 220 of 30 Mbps, and two VC 231 and 232 are tied up in a bundle within VP(2) 230 of 30 Mbps, respectively.
As to the contracts carried out in the VP units, the network management side monitors only whether each traffic keeps the contract bandwidth set for each VP. Namely, in FIG. 19, the UPC function of the ATM switch 341 located at the entrance of the public ATM network 340 monitors a contract violation in each VP unit over the line 200 and takes measures such as marking, cell discarding, etc. against each violated cell, but does not monitor each individual bandwidths in VC units bundled within each of said VPs.
When the violated cell is discarded owing to the action of the UPC function, the large delay occurs in the transfer of the information and the additional congestion occurs due to the re-sending of each cell as described above. It is therefore desirable that even each transmitting terminal is provided with a traffic shaping function (hereinafter called “VP shaping”) for sending cells while keeping the contracted VP bandwidth.
When, for example, a traffic shaping device or shaper of the terminal 300 or ATM switch 341 sends each received cell to its corresponding output line while a reception sequence is being kept as it is although the bandwidth for each VC in VP is not monitored on the network side, the assurance of the bandwidth for the real-time traffic for the voice and pictorial communications and the low delay transfer cannot be implemented as the entire network. A traffic shaping function (hereinafter called “VC shaping”) for effecting bandwidth control in VC units and priority control set according to the service classes on each individual VC bundled in one VP is required to transfer each cell at the real-time traffic with the low delay as in the case of the CBR and VBR services.
However, the prior art 2 and the prior art 3 have described the shaping function for either VP or VC and do not disclose, similar to the prior art 3, the traffic shaping technique which simultaneously perform shaping for both VP and VC.